This invention relates to coffee and more particularly to a percolation process wherein coffee solids are extracted from roasted and ground coffee to produce a coffee extract which is then further processed to form a dry soluble coffee product.
Percolation is the semi-continuous countercurrent extraction of water soluble coffee solids from roasted and ground coffee. Percolation is performed in a percolation set which is comprised of a series of extraction columns, generally 4 to 6.
At steady-state operation, the extraction columns of the percolator set are filled with roasted and ground coffee of varying degrees of extraction or freshness. An aqueous extraction liquid, generally water, is heated and fed to the entrance of the extraction column containing the most extracted roasted and ground coffee in the percolator set, generally known as the spent stage extraction column. The extraction liquid passes through the spent stage contacting the roasted and ground coffee contained therein and exits the extraction column as a dilute solution of coffee solids. The extraction liquid is then passed to and through the next successive extraction column containing the next most extracted coffee in the percolator set, extracting soluble solids therefrom. In like manner the extraction liquid is passed through successive extraction columns containing progressively fresher or less extracted roasted and ground coffee. The extraction liquid is finally passed through the extraction column containing the least extracted or freshest coffee in the set, generally unextracted coffee, known as the fresh stage extraction column and a predetermined portion of the extraction liquid is drawn off as coffee extract thus completing a cycle. This coffee extract generally contains from 20% to 35% soluble coffee solids by weight and is then further processed to produce a dry soluble coffee product.
A new cycle is begun by taking the spent stage extraction column off-stream and placing a new extraction column generally containing unextracted roasted and ground coffee on-stream thus becoming the fresh stage for draw-off of coffee extract in this cycle. Aqueous extraction liquid is then fed to the spent stage extraction column for this cycle, the coffee contained therein being the next most extracted coffee from the previous cycle, and extraction continues as described above with the extraction liquid contacting progressively fresher roasted and ground coffee.
It can be seen, then, that a given charge of roasted and ground coffee becomes progressively more extracted in each successive cycle.
There are numerous factors that affect the proper loading of a coffee column in preparation for extraction. Some are percolator wall temperature, rate of coffee fill into the percolator column, size and shape of ground coffee bins, uniformity of grinds, and slackness in percolator column fill.
Coarse particles in a free fall tend to roll to the edge of a bin. Fines and chaff settle later in the corners of square bins and at the edges of round bins. This phenomenon occurs in all the bins in which the ground coffee is handled, as well as in the percolator column under gravity fall of ground coffee. Faster filling of a bin or percolator column traps particles as they fall so segregation is minimized. Fast filling of the column is attained by sucking the ground coffee in with air and removing the air from inside the column. The fast fill has the advantage of giving higher coffee loadings in the column by about 10 per cent over gravity fill. Higher coffee column loadings mean higher productivity from the percolator system and a few percent higher extract concentrations. Denser column loadings reduce channeling of extract flow which is more important in larger diameter columns. Uniformly fine grinds also give less particle segregation and higher density of fills. Additionally, steam at about 90 p.s.i. to about 150 p.s.i. is employed to further compress or pack the coffee loaded within the columns. Steam packing allows additional coffee to be admitted to the column which is again steam packed. Roasted and ground coffee loaded into percolator columns is described by those skilled in the art as exhibiting a packed bed character.
A slack coffee bed results in mixing of extracts and channeling which reduces extraction efficiency and gives lower extract concentrations and lower solubles yields. The values of the snug high density vacuum fill of dry ground coffee are that the coffee bed is rigid, a swelling of 7 per cent after wetting further reduces void, and extract flow is through a uniformly supported bed and flow path. Prewetting of ground coffee before filling the column will reduce frequency of excessive pressure drop, but this type of slackness results in stale flavored coffee, looser beds with channeling and lower extract concentrations.
In commercial percolation processes extraction columns are completely filled with roasted and ground coffee and are sized such that the appropriate amount of coffee extract is produced. Further, commercial percolation processes are operated at cycle times, i.e., the time between successive draw-offs of coffee extract from the fresh stage, which are relatively short so as to more economically produce coffee extract. The coffee extract drying system is similarly fashioned to accommodate the maximum percolation output. The net result of this type of operation is a relatively fixed-capacity system. However, there often exists the need for operating a given percolator set at capacities below the maximum for which it was designed. It is further essential, of course, that the overall quality of the resultant coffee extract not be diminished.